A lens unit provided with a coming-off prevention structure of an O-ring has been disclosed in Japanese Patent Laid-Open No. 2011-059396. In the lens unit disclosed in the Patent Literature, an O-ring is attached to a small diameter part 11a of a lens 11 disposed on the most object side of a plurality of lenses and the O-ring is prevented from displacing to the image side by forming an outer peripheral face of the small diameter part 11a in an inclined face which is enlarged to an outer side in a radial direction toward the image side.
An on-vehicle periphery monitoring camera which is arranged on an outer side of a vehicle and is structured to capture images of the condition surrounding its vehicle is used under harsh environmental conditions such as wind, rain and dust. Therefore, respective units including the lens unit which constitute a monitoring camera are required to provide with a measure for preventing foreign matters from entering into an inside of the mechanism.
In order to provide a lens unit with waterproof performance by using an O-ring, in a case that the most object side lens (hereinafter, referred to also as “first lens”) is a glass lens, the lens unit may be structured as shown in FIG. 7A. In other words, an O-ring 4′ is attached to a lens holder in a state that upper and lower faces of the O-ring 4′ are compressed by the first lens 21′ in an axial direction.
In such a lens unit, generally, after respective lenses and an O-ring 4′ are disposed within a lens holder, a caulking part provided at an object side edge part of the lens holder is thermally caulked and thereby they are fixed in the inside of the lens holder. In this case, the object side edge part of the lens holder is required to thermally caulk while pressing the first lens 21′ against the lens holder with a jig so that the first lens 21′ is not lifted to the object side by a repulsive force of the O-ring 4′ which is compressed in the axial direction.
Since heat is transmitted to the jig when thermal caulking is performed, temperature of the jig may rise close to about 200 degrees (200° C.). The first lens 21′ is pressed by the jig at a high temperature and thus, in this method, it is a prerequisite that the first lens 21′ is a glass lens. When the first lens 21′ is a plastic lens, a portion of the plastic lens with which the jig is abutted will be melted and thus, this method cannot be used.
As a structure for attaching an O-ring 4′ to a lens unit whose first lens 21′ is a plastic lens, a structure is conceivable in which the first lens 21′ is not pressed by a jig as shown in FIG. 7B. In the lens unit shown in FIG. 7B, faces of the O-ring 4′ in the inside and outside diameters are compressed in a radial direction by the first lens 21′ and thus the first lens 21′ is not pressed to the object side by a repulsive force of the O-ring 4′. A plastic lens has larger flexibility in shape than a glass lens and thus the structure shown in FIG. 7B is commonly adopted.
An O-ring 4′ is assembled into the lens unit shown in FIG. 7B as follows. First, an O-ring 4′ is attached to a first lens 21′ and, after that, the first lens 21′ to which the O-ring 4′ is attached is fitted into the lens holder and a caulking part of the lens holder is thermally caulked. In this assembling method, an attaching position of the O-ring 4′ may be sometimes displaced when the first lens 21′ is press-fitted into the lens holder or due to unintended contacting of the O-ring 4′ with the lens holder or the like. Specifically, when the first lens 21′ to which the O-ring 4′ is attached is to be fitted into the lens holder, for example, in FIG. 7B, if a part on the left end side of the O-ring 4′ in the drawing is caught by a flat face of the lens holder and the O-ring 4′ is not smoothly inserted into the lens holder, a right end side of the O-ring 4′ may be pulled to an inner side in the radial direction and to a left lower side and may be bitten between a flat face on the lower side of the lens holder and an under face of the first lens 21′. Alternatively, when a distance between the under face of the first lens 21′ and a flat face on the lower side of the lens holder is large, the O-ring 4′ is easily come off. In other words, when a position of the O-ring 4′ is displaced, the O-ring 4′ may be come off or the O-ring 4′ may be bitten and caught between the first lens 21′ and the lens holder.
Further, the lens unit shown in FIG. 7B is structured so that the faces of the O-ring 4′ in the inside and outside diameters are compressed in the radial direction by the first lens 21′ and thus clearance in a groove where the O-ring 4′ is disposed is provided in an axial direction. From a viewpoint of assembling, a structure that a cylindrical surface is sealed by the faces in the inside and outside diameters of the O-ring may easily occur displacement of an attaching position of the O-ring in comparison with a case that sealing is performed by upper and lower faces of the O-ring. Therefore, even after lenses and an O-ring are assembled into a lens holder, an attaching position of the O-ring in the groove may be displaced through usage on an outer side of a vehicle under harsh environmental conditions. As a result, distortion, twisting, slacking or the like may occur in the O-ring and thus waterproof performance may be lowered.